Coupling control in side-polished fiber devices

ABSTRACT

Fiber optical devices formed on substrates fabricated with grooves that operate based on evanescent optical coupling through a side-polished fiber surface in each fiber involved. The fiber cladding under the side-polished fiber surface is designed to support a radial mode profile wider than a radial mode profile supported by adjacent fiber portions.

This application is a continuation-in-part of both U.S. application Ser.No. 09/905,038 entitled “DEVICES BASED ON FIBERS ENGAGED TO SUBSTRATESWITH GROOVES” filed on Jul. 12, 2001 and U.S. application Ser. No.09/796,373 entitled “INTEGRATION OF FIBERS ON SUBSTRATE WITH GROOVES”and filed on Feb. 27, 2001. This application also claims the benefit ofU.S. Provisional Application No. 60/230,820 entitled “A DIRECTIONALCOUPLER” and filed on Sep. 7, 2000.

BACKGROUND

This application relates to optical fiber devices, and morespecifically, to devices based on evanescent optical coupling betweentwo fibers.

Optical waves may be transported through optical waveguiding elements or“light pipes” such as optical fibers. A typical fiber may be simplifiedas a fiber core and a cladding layer surrounding the fiber core. Therefractive index of the fiber core is higher than that of the fibercladding to confine the light. Light rays that are coupled into thefiber core within a maximum angle with respect to the axis of the fibercore are totally reflected at the interface of the fiber core and thecladding. This total internal reflection provides a mechanism forspatially confining the optical energy of the light rays in one or moreselected fiber modes to guide the optical energy along the fiber core.Optical fibers may be used in transmission and delivery of opticalsignals from one location to another in a variety of optical systems,including but not limited to, fiber devices, fiber links and fibernetworks for data communications and telecommunications. In addition,optical fibers may be used to form various optical devices to modify,filter, or process guided optical energy.

The guided optical energy in a fiber, however, is not completelyconfined within the core of the fiber. A portion of the optical energycan “leak” through the interface between the fiber core and the claddingvia an evanescent field that essentially decays exponentially with thedistance from the core-cladding interface. The distance for a decay inthe electric field of the guided light by a factor of e≈2.718 is aboutone wavelength of the guided optical energy. This evanescent leakage maybe used to couple optical energy into or out of the fiber core, oralternatively, to perturb the guided optical energy in the fiber core.

SUMMARY

According to one embodiment, a fiber device includes a substrate havingfirst and second opposing substrate surfaces and including an elongatedgroove formed over the first substrate surface, and a fiber having afiber coupling portion engaged in the elongated groove. A portion offiber cladding of the fiber coupling portion is removed to form a sidefiber surface spaced from a fiber core of the fiber within a reach of anevanescent field of a guided mode in the fiber. The fiber couplingportion has a fiber cladding portion whose radial index distribution isdifferent from adjacent fiber portions to produce a radial mode profilewider than a radial mode profile of the adjacent fiber portions.

Methods for fabricating a fiber device are also provided. In oneembodiment, a method includes selecting a fiber portion of a fiber,removing a portion of fiber cladding of the selected fiber portion toform a fiber coupling surface spaced from a fiber core of the selectedfiber portion within a reach of an evanescent field of a guided mode,and modifying a property of said selected fiber portion. Themodification increases a cladding refractive index of the selected fiberportion and to increase a spatial mode profile of said guided mode inthe selected fiber portion to be greater than a spatial mode profile ofthe guided mode in an adjacent fiber portion whose cladding refractiveindex is not modified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of a fiber device that integrates or engagesa fiber to a substrate with a groove for positioning the fiber andopenings for holding the fiber.

FIGS. 2A and 2B show a cross sectional view of the device in FIG. 1along the direction AA′ and a side view of the device in FIG. 1 alongthe direction BB′, respectively.

FIGS. 3A and 3B show examples of two different cross sections forgrooves shown in FIG. 1.

FIGS. 4A, 4B, 5A, 5B, 5C, 5D, and 5E illustrate a process of fabricatingV grooves in semiconductor substrates by anistropic etching.

FIG. 6 illustrates formation of openings in V grooves by anistropcetching.

FIG. 7A shows a substrate that is fabricated with an array of grooveswith openings.

FIG. 7B shows a fiber device formed on a substrate with two or moregrooves aligned with each other along a straight line on a single sideof the substrate.

FIGS. 7C and 7D show fiber devices formed on a substrate with grooves ona single side of substrate that are oriented in different relativedirections.

FIGS. 8A, 8B, 8C, 8D, and 9 illustrate substrates that are processedwith grooves on both substrate surfaces.

FIG. 10 shows a substrate processed with grooves on both substratesurfaces, where openings at both ends of each groove are separatelyfabricated from the V grooves.

FIGS. 11 and 12 show exemplary fiber devices by integrating fibers tosubstrates with grooves.

FIG. 13A shows uses of relative positions between grooves to controloptical coupling between fibers positioned in the grooves.

FIG. 13B shows a substrate with both deep and shallow grooves formed ona single side.

FIG. 13C shows a substrate with both deep and shallow grooves formed onboth sides.

FIG. 14 shows an exemplary fiber device that has lateral jump-channelgrooves on the substrate to change a direction of a fiber in thesubstrate plane.

FIG. 15 shows a design to engage a side-polished fiber onto a substrateby using an elongated groove with a single through hole.

FIG. 16 shows one embodiment of a fiber coupler based on evanescentcoupling between two side-polished fibers engaged to two differentsubstrates.

FIG. 17 illustrates a cross sectional view of the fiber coupling regionalong lines AA of the device in FIG. 16.

FIG. 18 illustrates spatial expansion of a guided mode in a fiber due toincreasing the cladding index of the fiber.

FIG. 19 shows the index modification process according to oneembodiment.

FIG. 20 shows a gradual profile along a longitudinal direction of thefiber of the cladding index as a result of the index modification.

FIG. 21 shows that the radial mode profile of a fiber, after the indexmodification, varies along the fiber from one side of the side-polishedfiber coupling port to the other side.

FIG. 22 shows a use of radiation exposure to modify the cladding indexin a fiber that has a radiation-sensitive cladding material.

FIG. 23 shows an ion,exchange process to modify the cladding index in afiber.

FIG. 24 shows a grating formed in the fiber coupling port of aside-polished fiber engaged to a substrate.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

The devices and techniques of this application use at least one fiberintegrated on or engaged to a substrate fabricated with one or moregrooves. One portion of the cladding of this fiber is removed andpolished to form a fiber coupling port. The fiber coupling port has asurface that is sufficiently close to the fiber core so that opticalenergy can be coupled through via evanescent fields out of or into thefiber core.

The following first describes in detail the basic structures forintegrating or engaging a fiber to a substrate to form at least onefiber coupling port in such a fiber. Next, techniques for modifying therefractive index of the fiber cladding are disclosed to improve thefabrication tolerance in the spacing between two coupled fibers.

FIG. 1 shows one embodiment of a fiber device 100 where a fiber 140 isintegrated or engaged to a substrate 110. The fiber device 100 may beused as a building block to construct a variety of fiber devices,including but not limited to, fiber couplers, fiber attenuators, fibermodulators, fiber beam splitters, optical fiber switches, and fiberfrequency-division multiplexers. FIGS. 2A and 2B show additional detailsof the fiber device 100.

The substrate 110 may be formed of various materials, such assemiconductors, insulators including dielectric materials (e.g., aglass, a quartz, a crystal, etc), metallic materials, or any othersolid-state materials that can be processed to form the device featuressuch as grooves and through holes disclosed herein. Two parallel andopposing substrate surfaces, 112 and 114, are generally flat and may bepolished. An elongated groove 120 is formed in the substrate 110 on thesurface 112 and is essentially a recess from the surface 112. The groove120 may be fabricated by removing a portion of the material from thesubstrate 110 through etching or other processes.

The geometry of the groove 120 is generally elongated along a straightline as illustrated or along a curved line. Unless otherwise indicated,the following description will use straight-line grooves as examples.Some embodiments are described with specific reference to groove withV-shaped cross sections as shown by the groove 310 in FIG. 3B. The crosssections are generally not so limited and may also be other shapes aswell, including rectangular as shown in FIG. 2A, U-shaped as shown bythe groove 310 in FIG. 3A, a circularly shape or other suitable shapes.

The width, W, of the groove 120 is generally greater than the diameter,d, of the fiber 140 and may either remain a constant or vary spatiallyalong the groove 120, e.g., increasing from the center towards the twoends. The length, L, of the groove 120 may vary from one grove toanother and can be determined based on specific requirements ofapplications. The depth D of the groove 120 may be a constant or mayvary along the groove 120, e.g., increasing from the center towards thetwo ends. In general, at least a portion of the groove 120 has a depth Dto expose a portion of the fiber cladding of the fiber 140 above thesurface 112 while still keeping the fiber core below the surface 112.Sometimes, the depth D of the groove 120 may also be selected to exposethe fiber core. Other portions of the groove 120 may have a differentdepth so that the fiber can be placed within the groove 120 under thesubstrate surface 112. Depending on the geometry of the groove 120(e.g., the apex angle of a V-shaped groove), the depth D of the entiregroove 120 may be greater than fiber diameter d. For a groove with arectangular cross section as shown in FIG. 2A, at least a portion of thegroove 120 has a depth D less than the fiber diameter d but greater thanthe sum of the fiber radius r=d/2 and radius of the fiber core rc=dc/2.This portion of the groove 120 exposes partial fiber cladding of thefiber 140 above the surface 112 while still keeping the fiber core belowthe surface 112. Other portions of the groove 120 may have a depth thatis at least the fiber diameter d so that the fiber can be essentiallyplaced in the groove 120 below the surface 112. However, in certainapplications such as the device shown in FIG. 12, the depth D of theentire groove 120 may be greater than fiber diameter d. Unless otherwiseindicated, the following description will assume that at least a portionof a groove 120 to expose a portion of the fiber cladding above thesurface 112 and adjacent portions sufficiently deep to keep the fiberbelow the surface 112. In case of the rectangular groove 120, thecentral portion of the groove 120 may have a depth D less than d butgreater than (d+dc)/2 while the portions on either sides of the centralportion may have a depth equal to or greater than the fiber diameter d.

Notably, the fiber device 100 includes two openings 131 and 132 that arerespectively formed at the two ends of the groove 120 and penetratethrough the substrate 110. Hence, the openings 131 and 132 are throughholes extending between the two surfaces 112 and provide access from onesurface (112 or 114) to another. The spacing between the openings 131and 132 essentially determines the length L of the groove 120. Theaperture of the openings 131 and 132 should be sufficiently large toreceive the fiber 140, e.g., with a diameter greater than the diameterof the fiber 140. The shape of the holes 131 and 132 may generally be inany suitable geometry.

A portion of the fiber 140 is placed in the groove 120 near the surface112. The remaining portions 141, 142 of the fiber 140 on both sides ofthe portion in the groove 120 are respectively fed through the first andsecond openings 131, 132 to the other side 114 of the substrate 110.After being placed in the substrate 110 as shown in FIG. 1, the fiber140 may be slightly pulled by moving the fiber portions 141 and 142 inopposite directions so that the portion of the fiber 140 in the groove120 is in substantially full contact with the groove 120.

Since a portion of the groove 120 has a depth D less than the fiberdiameter d, the cladding of the fiber 140 in this portion protrudes outof the surface 112. The fiber core in this portion of the fiber isgenerally kept under the surface 112. For example, the cladding of acentral portion of the fiber 140 between the holes 131 and 132 may beexposed. This protruded or exposed cladding is then removed and polishedto form a flat surface 144 of a length Lc that is above the fiber core143 and is substantially coplanar with the surface 112 of the substrate110. When the spacing, h, between the flat surface 144 and the fibercore 142 is sufficiently small (e.g., on the order of or less than onewavelength of optical energy), the flat surface 144 can be used tocouple optical energy into or out of the fiber core 144 through theevanescent fields outside the fiber core. Hence, the length, Lc, of theflat surface 144 approximately represents the optical coupling lengthfor the fiber device 100. This coupling surface 144 may also benon-flat, e.g., curved to a certain extent, as long as it can transmitevanescent signals.

FIGS. 4A and 4B illustrate the fabrication of the V groove 320 andplacement of the fiber 140 in the V groove 320 as shown in FIG. 3B.First, a mask layer 410 is deposited over the surface 112 of thesubstrate 110 and is patterned by a suitable technique such as aphotolithography process to have one or more groove areas exposing theunderlying substrate 110. Next, the exposed portions of the substrate110 are anistropically etched to form V grooves.

If the substrate 110 is formed of a semiconductor, e.g., silicon, athermally-grown silicon oxide or nitride film may be used as the etchingmask 410 for anisotropic silicon etching. When the surface 112 is in thecrystalline plane (100) of the Si substrate 110 and the groove patternsin the etching mask 410 are parallel to the crystalline plane (110), anetchant chemical such as alkaline (KOH) can be applied on the silicon(100) surface to produce truncated v-shaped grooves. Since theanisotropic etching is stopped at the crystalline plane (111), thedimension of the V grooves, such as the groove width and depth can beaccurately controlled by properly selecting the dimension of the groovepatterns formed in the etching mask 410.

Referring to FIG. 4B, after the grooves 320 are formed, the fibers 140can be placed in the grooves 320 and bonded to the groves 320 atlocations 420. The bonding may be implemented by a number of techniques,including but not limited to using an epoxy, glass frit thermal bond, orCO2 assisted thermal bond. When multiple grooves 320 are formed, anarray of fibers 140 can be precisely aligned in the grooves 320 with apredetermined spacing. The exposed cladding of the fiber 140 can then beremoved and polished to form the flat surface 144 as shown in FIG. 3B.

FIG. 5A shows one exemplary groove pattern 500 formed in the etchingmask layer 430 in FIG. 4A. FIG. 5B illustrates the corresponding Vgroove 320 in the silicon substrate 110 formed from the anistropicetching by using the mask 500. The opening of the groove pattern 500 isdesigned to gradually widen from the center to both sides along thegroove to be formed. Accordingly, the width and depth of the underlyingV groove 320 also increase from the center portion 510 to side portions520 that are spaced from the center along the groove 320. Asillustrated, the surfaces of the V groove 320 are not flat but arecurved as a result of etching through the above mask 500. FIGS. 5C, 5D,and 5E show the placement of fibers 140 in the above V-groove structure.

The above anistropic etching may be used to form both the V groove 320and the openings 131 and 132 at both sides of the V groove 320 as shownin FIG. 1. Referring to FIG. 6, when opening of the groove pattern 500in the etching mask 410 is sufficiently wide, side portions 620 of the Vgroove 610 can extend all the way through the substrate 110 from thesurface 112 to the opposite surface 114 and hence create an opening 620on the surface 114. The openings 620, therefore, can be used as theopenings 131 and 132 to allow the fiber 140 to go through the substrate110 from the surface 112 to the opposite surface 114.

FIGS. 7A and 7B show that an array 700 of such V grooves 710 with twoopenings can be formed on one side of the substrate 110. The V grooves710 may be aligned to be parallel to one another along their elongateddirections and are arranged to form multiple parallel columns 730 with aspacing 740. Within each column 730, multiple V grooves 710 may bespaced from one another by a spacing 720. The substrate 110 with thearray 700 may diced into multiple units each having one or more Vgrooves 710. Such units can be used to form various fiber devices.Hence, a batch fabrication process may be used to process the substrate110 and to simultaneously form multiple fiber devices with V grooves710.

A single fiber can be threaded through different V grooves 710 in acolumn 730 between the surfaces 112 and 114 via the openings 131 and132. FIG. 7B shows an example where the fiber 140 is threaded through Vgrooves 710A, 710B, 710C, and 710D formed along a straight line on thesurface 112 of the substrate 110. A spacer 721, such as a rod, may beoptionally positioned on the surface 114 between the openings of twoadjacent V grooves to provide a support to the fiber 140. Such supportmay be used to reduce sharp bending of the fiber 140 which may damagethe fiber 140. After bonding and polishing the fiber 140, a couplingport is formed at each V groove on the surface 112 and is operable tocouple optical energy out of or into the fiber 140. Therefore, thisdevice has multiple coupling ports on the surface 112 to couple opticalenergy into or out of the fiber 140. When a proper control mechanism isimplemented at each coupling port, optical switching, opticalmultiplexing, and other coupling operations may be achieved.

FIGS. 7C and 7D show additional embodiments of fiber devices that twodifferent grooves 750 and 760 on the substrate 110 are not aligned alonga straight line as in FIGS. 7A and 7B but form an angle with respect toeach other. Numerals 751, 752, 761, and 762 indicate the openings of thegrooves 750 and 760 that penetrate through the substrate 110. In FIG.7C, the two grooves 750 and 760 are spaced from each other. A fiber maybe placed in the grooves 750 and 760 by sequentially passing the fiberthrough the openings 761, 762, 752, and 751. In FIG. 7D, two grooves 750and 760 are share a common opening 752. Such arrangements may becombined with aligned grooves.

Referring back to FIG. 1, the groove 120 with its two openings 131 and132 may be formed on both sides 112 and 114 of the substrate 110 in thefollowing manner. First, two adjacent grooves respectively formed indifferent sides of the substrate are aligned along the same groovedirection. Second, the groove on one side shares an opening with theadjacent groove on the opposite side of the substrate 110. Techniquessuch as the double-sided photolithography may be used to form the Vgrooves on both surfaces of the substrate. Unlike the fiber device shownin FIG. 7B where the.coupling ports are only on a single side of thesubstrate, a substrate with V grooves on both sides can form a fiberdevice with coupling ports on both sides of the substrate. Suchdouble-sided coupling capability can provide flexible and versatilecoupling configurations in various fiber devices.

FIGS. 8A, 8B, and 8C illustrate one example of a fiber device 800 thathas V grooves on both sides 112 and 114 of the substrate 110. A first Vgroove 820 is formed on the side 114. Similar to the V grooves in FIGS.5B and 6, the depth and width of the V groove 820 increase from itscenter towards both ends 820A and 820B. A second, similar V groove 810is formed on the opposite side 112 along the same groove direction. Theend 810A of the second groove 810 overlaps with the end 820A of thefirst V groove 820 to create a through hole 812 that connects the Vgrooves 810 and 820. A third V groove 830 is also shown on the side 112to have one end 830A overlap with the end 820B of the V groove 820 onthe opposite side 114. A through hole 822 is then formed at theoverlapping region to connect the V groove 820 to the V groove 830. Afiber 140 is shown in FIG. 8C to thread through the holes 812 and 822 toform coupling ports on both sides 112 and 114 of the substrate 110.

FIG. 8D shows a 3-port fiber device 840 that is formed by dicing alinear array of V grooves 810, 820, and 830 from the substrate 110.Comparing to the single-side device shown in FIG. 7B, the naturalcurvature of the V grooves formed on both sides eliminates the spacers740. Similar to the batch fabrication of the single-sided devices shownin FIG. 7A, multiple double-sided devices may also be simultaneouslyfabricated from a single-sided substrate as illustrated in FIG. 9.

In the above devices with V grooves formed on both sides of thesubstrate, two adjacent V grooves, located on opposite sides of thesubstrate, may not be aligned along a straight line but form an anglewith each other as illustrated by the adjacent grooves formed on thesame side shown in FIGS. 7C and 7D. Similar to the grooves in FIGS. 7Aand 7B, two adjacent V grooves, located on opposite sides of thesubstrate, may also be designed to spatially separate from each otherwithout sharing a common opening that penetrates through the substrateand extends between two sides of the substrate.

The openings in the above examples of V grooves are formed byanistropically etching for forming the V grooves. Hence, there is noneed to use a separate process to fabricate the openings if the etchingmask is properly designed. However, a separate fabrication step may alsobe used to form an opening and to achieve any desired geometric shape ofthe opening that may be difficult or impossible to make through etchingthe V grooves.

FIG. 10 illustrates a fiber device 1000 with aligned V grooves 810, 820,and 830 on both sides 112 and 114 of the substrate 110 that are spacedfrom one another by rectangular openings 1010 and 1020. The V grooves810 and 830 are formed on the side 114 and the groove 820 is formed onthe opposite surface 112 but is located between the grooves 810 and 830.An etching process separate from etching of the V grooves is needed toform such openings 1010 and 1020. Other processing techniques such aslaser machining may also be used to form the openings.

The above fiber devices with V grooves either on one side or two sidesmay be used to form various fiber devices. Some exemplary devices aredescribed below.

FIG. 11 shows an optical fiber coupler 1100 by using two substrates 1110and 1120 each with V grooves on a single surface of the substrate. Thesubstrate 1110 has a surface 1110A on which three V grooves arefabricated and a fiber 140A is placed therein to form three couplingports 1111, 1112, and 1113. Similarly, the substrate 1120 has a surface1120A on which three V grooves are fabricated and a fiber 140B is placedtherein to form three coupling ports 1121, 1122, and 1123. The twosubstrates 1110 and 1120 are engaged by having the surfaces 1110A and1120A to face each other. The ports on one substrate substantiallyoverlap with the coupling ports of another substrate to allow energyexchange between the fibers 140A and 140B. Various techniques may beused to engage the two substrates together, such as optical epoxy, glassfrit thermal bond, CO2 laser assisted thermal bond.

A fiber device with V grooves on both sides of the substrate can be usedto provide coupling on both sides. More coupling flexibility can beachieved in such a device than a device with grooves on only one side.For example, each fiber in the device 1100 shown in FIG. 11 cannot beaccessed from the exposed surfaces 1110B and 1120B. Such access would bepossible if one of the two substrates 1110 and 1120 were designed tohave grooves on both sides. Thus, three or more substrates may bevertically stacked together to form a multi-layer optical coupler. Sinceeach substrate may have two or more fibers, coupling among many fibersin different substrates may be achieved.

FIG. 12 shows a 4-layer optical multi-port coupler 1200 having 4different double-sided substrates 1201, 1202, 1203, and 1204 based onthe designs shown in FIGS. 8D or 10. Four different fibers 1210, 1220,1230, and 1240 are respectively threaded in the substrates 1201, 1202,1203, and 1204. Two adjacent substrates, such as 1201 and 1202, may becoupled to form the coupling ports 1212, 1214, and 1216. Hence, opticalenergy can be coupled between any two fibers. For example, an opticalsignal in the fiber 1210 may be coupled to the fiber 1230 by firstcoupling into the fiber 1220 and then coupling from the fiber 1220 intothe fiber 1230. In general, a double-sided substrate can interface atboth sides with other single-sided or double-sided substrates.

FIG. 13A illustrates that optical coupling between two fibers indifferent layers may be controlled in a number of ways by controllingthe relative position of the two fibers in grooves. For example, nooptical coupling occurs between fibers 1301 and 1302 in the layers 1201and 1202 when they are placed in deep grooves to have a separation muchgreater than one wavelength of the light. The fibers 1303 and 1304 inthe layers 1202 and 1203 are positioned in shallow grooves so that aportion of each fiber's cladding is removed to allow for opticalcoupling. The depth of the grooves for the fibers 1303 and 1304 can becontrolled to control the coupling strength via evanescent fields. Thefibers 1305 and 1306, also in shallow grooves, are spatially offset inthe lateral direction so that the optical coupling is reduced with,theamount of the offset.

The grooves for holding fibers 1301 and 1302 are “deep” grooves in thatthe depth of the groove is greater than the diameter of the fiber sothat the fiber cladding in the fiber portion in such grooves is notexposed above the substrate surface and no optical coupling port isformed. The grooves for holding the fibers 1303, 1304, 1305, and 1306,on the other hand, are “shallow” grooves as the groove 120 describedwith reference to FIG. 1 where a portion of a part of the fiber claddingprotrudes above the substrate surface when the fiber is placed in such agroove and can be removed to form an optical coupling port 144. Suchdeep and shallow grooves may be combined to provide flexibility andversatility in routing fibers and arranging optical coupling ports in afiber device.

FIG. 13B shows a single-sided substrate similar to the substrate in FIG.7B but processed to have both deep grooves 1312 and shallow grooves1310. Each deep grove 1312 is used at a location where optical couplingis undesirable. FIG. 13C shows a double-sided substrate with deepgrooves 1330 and shallow grooves 1320.

FIG. 14 further shows that a lateral jump-channel groove 1424 on asubstrate 1400 may be used to change the lateral direction of a fiber.The substrate 1400 is shown to have grooves on both sides. Solidelongated boxes such as 1410 represent grooves formed on one side andthe dashed elongated boxes such as 1412 represent grooves formed on theother side. The grooves 1410, 1412, 1414, 1416, and 1418 are alignedwith one another along a straight line to hold a fiber 1401. The groove1424 is a lateral jump-channel groove that is oriented with an anglerelative to adjacent grooves 1422 and 1436. Hence, a fiber 1402 can bethreaded through the lateral jump-channel groove 1424 to run throughgrooves 1440 and 1422 and then to change its direction to run throughgrooves 1436 and 1438. Lateral jump-channel grooves 1432 and 1444 arealso shown to direct the fiber 1402 from the groove 1430 to grooves 1456and 1458. A single-side substrate with grooves on one side may also bedesigned to have such lateral jump-channel grooves.

Such a lateral jump-channel can be combined with the verticalintegration of different double-side substrates to change the directionof an optical signal both laterally within a substrate and verticallyfrom one substrate to another substrate. This opens up possibilitysimilar to multi-layer printed circuit board technology allowingsophisticated connections from point to point and from layer to layer.

FIG. 1 shows the use of elongated groove 120 and two through holes 131and 132 at the ends of the groove 120 to engage a fiber to the substrate110 and to form each fiber coupling port 144. Alternatively, only onethrough hole 132 in the substrate 110 may be used to engage the fiber140 to form a fiber coupling port 144. FIG. 15 shows an example of sucha fiber structure 1500. The groove 120 may extend to one end side 1510of the substrate 110 so that one end 141 of the fiber 140 leaves thegroove 120 without going through a through hole.

FIG. 16 shows a fiber coupler 1600 that couples two side-polished fibers140A and 140B respectively engaged to two substrates 110A and 110B. Bothfibers 140A and 140B are polished to remove a portion of the fibercladding and to form the fiber coupling ports 144A on the substratesurface 112A and 144B on the substrate surface 112B, respectively. FIG.17 shows the crosssectional view of the engaged coupling ports 144A and144B along the line AA shown in FIG. 16. In order to allow forevanescent coupling between the fibers 140A and 140B, the spacing Dbetween the fiber cores 143A and 143B of the fibers 140A and 140B shouldbe set to place one fiber core in the evanescent field of a guidedoptical wave in another fiber core. Depending on the desired couplingstrength, this spacing D may be on the order of one wavelength of theoptical signal. For typical wavelengths at about 1.5 micron, the spacingD is generally less than a few microns.

In actual fabrication, the fiber coupling ports 144A and 144B may beformed by using a polishing process. However, in some polishingprocesses, it may be difficult to accurately control this spacing D ofless than a few microns between the fiber cores 143A and 143B of thefibers 140A and 140B. Hence, it is desirable to relax the requiredaccuracy in D in fabrication without significantly compromising desiredcontrol in the evanescent coupling strength.

One aspect of the present invention is to modify the refractive index ofthe fiber cladding in a selected fiber portion of at least one of thefibers 140A and 140B to expand the radial spatial extend of the guidedmode. The radial direction is substantially perpendicular to thelongitudinal direction of the fiber. FIG. 18 illustrates a radial modeprofile 1810 of a mode prior to the modification that is similar to themode in a conventional fiber and a radial profile 1820 of the expandedradial mode profile 1820 after the cladding index has been increased. Inparticular, the refractive index of the fiber cladding near the fibercore is increased to be closer to the index of the fiber core. Thiseffectuates an increase in the mode size. As a result, the spacing D canbe increased while still maintaining the evanescent couplingconfiguration between the fibers 140A and 140B. This increased spacing Dallows the D to vary in a larger tolerance range in comparison with thetolerance range for D prior to the modification of the cladding indicesof the two fibers 140A and 140B.

FIG. 19 shows the fabrication process of the fibers 140A and 140B.First, a single-mode fiber is provided to have a numerical aperture thatis substantially uniform along the fiber and is approximately equal tothat of commercial single-mode fibers. The radial mode profile of thefiber is small as shown by the mode 1810 in FIG. 18. Next, the fibercladding of a selected fiber portion is treated to increase the claddingindex and hence to achieve the spatially expanded mode as shown by themode 1820 in FIG. 18 in the treated fiber portion. Notably, the claddingindex in the selected fiber portion is modified to have a gradualprofile that changes gradually along the longitudinal direction of thefiber so that this no abrupt change in the cladding index along thefiber to cause adverse optical reflection along the fiber. FIG. 20 showsone example profile 200 after the index modification. As illustrated,the cladding index is high at center of.the fiber coupling port 144 (butstill less than the core refractive index) and gradually decreasestowards both ends of the fiber coupling port 144. This trend continuesalong the fiber until the cladding index of at the edge of the modifiedregion eventually becomes equal to the cladding index of adjacent,unmodified fiber portions.

After the index modification, the modified fiber has a varying radialmode profile from one side of the side-polished fiber coupling port tothe opposite side because only the fiber cladding in the selected fiberportion, which is in and adjacent to the coupling port 144, is modifiedand other fiber portions are not so modified. FIG. 21 shows that thefiber 140 has a narrow radial mode profile 1810 as in a conventionalfiber in the fiber portion 2101, an expanded radial mode profile 1820 inthe fiber coupling port 144, and the narrow radial mode profile 1810 inthe fiber portion 2102. Notably, the cladding-modified region withexpanded radial mode profile 1820 may be limited in one or a fewselected regions in the fiber to reduce the overall optical loss. Thisis because the expanded radial mode profile 1820 can benefit efficiencyof the evanescent coupling and the control of the coupling efficiencybut at the same time it can also introduce undesired optical loss due tothe expanded mode size in fiber portions where the evanescent couplingis not needed. Other fiber portions, which constitute the majority ofthe fiber, may preferably have the narrow radial mode profile thatapproximately matches that of conventional single-mode fibers to allowfor efficient interconnection with other fibers by using the end-to-endpigtail fiber connections. In the fiber device 1600 that couples twofibers 140A and 140B as in FIG. 16, it may be sufficient to have onlyone of the fibers 140A and 140B to be modified to have a narrower radialmode profile in the coupling port as shown in FIG. 21 while the otherfiber is a conventional fiber with a substantially uniform radial modeprofile. Alternatively, both fibers 140A and 140B may be made to havethe spatial mode distribution as in FIG. 21.

One way to modify the cladding index is to use a radiation-sensitivematerial to form the fiber cladding in at least the selected fiberportion which is to be processed to form the side-polished fibercoupling port. For example, the fiber cladding may be UV-sensitive andresponsive to the UV light to increase its index. FIG. 22 shows that afiber 140 may be first engaged to the substrate 110 and polished to formthe fiber coupling port 144. Then, the UV light 2200 is directed toilluminate the fiber portion that includes the fiber coupling port 144to increase the cladding index. The intensity of the UV light 2200 maybe controlled to have a spatially varying profile along the fiber 140:the UV light intensity is at a maximum value in the center of the fibercoupling port 144 and gradually decays along the fiber towards both endsof the port 144. In another implementation of the UV exposure, the fibercoupling region 144 may be first exposed to the UV light 2200 prior tobeing engaged to the substrate 110 and polished to form the port 144.

FIG. 23 shows an alternative technique where an ion exchange process maybe performed to increase the index of the fiber cladding. First, thefiber 140 is engaged to the substrate 110 and is then polished to formthe port 144. The ion exchange is then performed to implant ions in thecladding of the port 144 to increase its index. The density of theimplanted ions can be controlled to achieve the desired graduallongitudinal profile as shown in FIG. 20 in the cladding index to reduceundesired optical reflection.

FIG. 24 further shows that, a grating 2400 may be fabricated in thefiber coupling port 144 whose fiber cladding has been modified toachieve the expanded radial mode profile. This grating 2400 may beformed in the fiber core or in the fiber cladding. In operation, thegrating 2400 may be used to select a particular wavelength forevanescent coupling. The grating 2400 may be a tunable grating to adjustthe coupling at the port 144.

In the above devices, at least one buffer layer of a suitable materialsuch as a dielectric material like silicon dioxide or silicon nitridemay be formed over a groove under the fiber. This buffer layer may bedesigned to have certain mechanical or thermal properties to stabilizethe structure formed by the substrate, the buffer layer, and the fiberby reducing the mechanical or thermal stress between the siliconsubstrate and the glass fiber. Therefore the reliability of the devicecan be improved. For example, if the substrate is formed of silicon, adielectric material with a coefficient of thermal expansion (CTE)between the CTE values of the silicon and the glass fiber may be used asthe buffer. Two or more buffer layers may also be used to achievedesired stabilizing effects.

Only a few embodiments are disclosed. However, it is understood thatvariations and enhancements may be made without departing from thespirit of and are intended to be encompassed by the following claims.

What is claimed is:
 1. A device, comprising: a first substrate having afirst elongated groove formed over a first substrate surface; a firstfiber having a fiber coupling portion engaged in said first elongatedgroove, a portion of fiber cladding of said fiber coupling portion beingremoved to form a first side fiber surface spaced from a fiber core ofsaid first fiber within a reach of an evanescent field of a guided modein said first fiber; a second substrate having a second elongated grooveformed over a second substrate surface, said second substrate facingsaid first substrate surface to align said second elongated groove withsaid first elongated groove; a second fiber having a fiber couplingportion engaged in said second elongated groove, a portion of fibercladding of said fiber coupling portion being removed to form a secondside fiber surface spaced from a fiber core of said second fiber andwithin a reach of an evanescent field of a guided mode in said secondfiber, said second side fiber surface being within a reach of saidevanescent field of said guided mode in said first fiber, wherein saidfiber coupling portion in said first fiber has a fiber cladding portionwhose radial index distribution is different from adjacent fiberportions to produce a radial mode profile wider than a radial modeprofile of said adjacent fiber portions.
 2. The device as in claim 1,wherein a difference between a core refractive index and a claddingrefractive index at a boundary of a core and a cladding in said fibercoupling port is less than a difference between a core refractive indexand a cladding refractive index at a boundary of a core and a claddingin said adjacent fiber portions.
 3. The device as in claim 2, whereinsaid difference between a core refractive index and a claddingrefractive index at a boundary of a core and a cladding in said fibercoupling port changes gradually with a position along said fiber.
 4. Thedevice as in claim 1, further comprising an optical grating formed insaid first fiber coupling portion.
 5. The device as in claim 4, whereinsaid grating is formed in a fiber cladding.
 6. The device as in claim 4,wherein said grating is formed in a fiber core.
 7. The device as inclaim 4, further comprising a buffer layer between each fiber and arespective substrate to reduce a stress between said each fiber and saidrespective substrate.
 8. The device as in claim 1, wherein said fibercoupling portion in said second fiber has a fiber cladding portion whoseradial index distribution is different from adjacent fiber portions toproduce a radial mode profile wider than a radial mode profile of saidadjacent fiber portions.
 9. The device as in claim 1, wherein one ofsaid first and said second substrates includes a through hole betweensaid first substrate surface and a second, opposing substrate surfacethat penetrates through said substrate, said through hole located at oneend of a respective elongated groove and one section of a respectivefiber passing through said through hole to place another section of saidfiber over said second substrate surface.
 10. A method, comprising:selecting a fiber portion of a fiber; removing a portion of fibercladding of said selected fiber portion to form a fiber coupling surfacespaced from a fiber core of said selected fiber portion within a reachof an evanescent field of a guided mode in said selected fiber portion;and modifying a property of said selected fiber portion to increase acladding refractive index of said selected fiber portion and hence toincrease a spatial mode profile of said guided mode in said selectedfiber portion to be greater than a spatial mode profile of said guidedmode in an adjacent fiber portion whose cladding refractive index is notmodified.
 11. The method as in claim 10, wherein said selected fiberportion has a fiber cladding that is radiation sensitive, and where saidmodifying includes exposing said selected fiber portion to a radiationbeam to increase said cladding refractive index.
 12. The method as inclaim 10, wherein said modifying includes implanting ions into the fibercladding of said selected fiber portion.
 13. The method as in claim 10,further comprising engaging said fiber coupling surface of said fiber toa fiber coupling surface of another fiber to cause evanescent couplingbetween the fibers.
 14. The method as in claim 10, further comprisingcontrolling said modification of said property to cause said claddingrefractive index to change gradually along the fiber.
 15. The method asin claim 14, wherein said cladding refractive index in said selectedfiber portion is modified to be greater than a cladding refractive indexof two fiber portions adjacent to said selected fiber portion but lessthan a core refractive index of said fiber at each location, and whereinsaid cladding refractive index of said selected fiber portion graduallydecreases along said fiber from a center of said selected fiber portiontowards two sides of said selected fiber portion and becomes equal tosaid cladding refractive index of said two fiber portions adjacent tosaid selected fiber portion.
 16. A device, comprising: a substratehaving first and second opposing substrate surfaces and including anelongated groove formed over said first substrate surface; and a fiberhaving a fiber coupling portion engaged in said elongated groove, aportion of fiber cladding of said fiber coupling portion being removedto form a side fiber surface spaced from a fiber core of said fiberwithin a reach of an evanescent field of a guided mode in said fiber;wherein said fiber coupling portion has a fiber cladding portion whoseradial index distribution is different from adjacent fiber portions toproduce a radial mode profile wider than a radial mode profile of saidadjacent fiber portions.
 17. The device as in claim 16, wherein saidsubstrate further includes a through hole between said first and saidsecond substrate surfaces that penetrates through said substrate, saidthrough hole located at one end of said elongated groove and one sectionof said fiber passing through said through hole to place another sectionof said fiber over said second substrate surface.
 18. The device as inclaim 16, further comprising a buffer layer in said elongated groovebetween said fiber and said substrate to reduce a stress between saidfiber and said substrate.
 19. The device as in claim 16, furthercomprising an optical grating formed in said fiber coupling portion. 20.The device as in claim 16, wherein a fiber cladding portion in saidfiber coupling portion has a refractive index greater than a claddingrefractive index of two fiber portions adjacent to said fiber couplingportion but less than a core refractive index of said fiber at eachlocation, and wherein said refractive index of said fiber claddingportion in said fiber coupling portion gradually decreases along saidfiber from a center of said fiber coupling portion towards two sides ofsaid fiber coupling portion and becomes equal to said claddingrefractive index of said two fiber portions adjacent to said fibercoupling portion.